Isomerase Isomerization reaction

Chemical reactions between biochemical compounds are enhanced by biological catalysts called enzymes, which consist mostly or entirely of globular proteins. In many cases a cofactor is needed to combine with an otherwise inactive protein to produce the catalytically active enzyme complex. The two distinct varieties of cofactors are coenzymes, which are complex organic molecules, and metal ions. Enzymes catalyze six major classes of reactions 1) Oxidoreductases (oxidation-reduction reactions), 2) Transferases (transfer of functional groups), 3) Hydrolases (hydrolysis reactions), 4) Lyases (addition to double bonds, 5) Isomerases (isomerization reactions) and 6) Ligases (formation of bonds with ATP (adenosine triphosphate) cleavage) [1]. 

Isomerases catalyse reactions such as cis-trans isomerization or more complex transformations such as D-glucose to D-fructose. 

However, the applicability of this strategy is limited by the substrate specificity of the isomerases so that only a fraction of the ketoses that can be obtained from the aldose-catalyzed reaction can be enzymatically isomerized to the corresponding aldose. Moreover, the isomerization reaction is reversible and, as a ketone is generally more stable than an isomeric aldehyde, the equilibrium produces substantial aldose isomer only if the aldose sugar can exist in a very stable aldopyra-nose form [38b]. 

Finally, the folding pathways of a number of proteins require two enzymes that catalyze isomerization reactions. Protein disulfide isomerase (PDI) is a... 

A great deal of research has been undertaken to determine if the dehydrogena-tion/isomerization reactions are properties of one system or of two separate enzymes (see Ref. 52) most of the evidence suggests that the former is true. Probably three, or even four, substrate-specific isomerases may occur in bovine adrenal cortex which can act on C19, C21 and C27 steroids. Likewise, separate 5-ene-3/3-HSDs may exist in the adrenal cortex for C19 and C21 steroids, because the latter did not compete with C19 steroids for active sites of the enzymes studied. 

The kinetics of the D-glucose-D-fructose isomerization reaction catalyzed by soluble D-glucose isomerase at a specified concentration of substrate can be related to a simple concept of an enzyme-catalyzed, reversible reaction. 

The rate of the isomerization reaction, catalyzed by a given quantity of D-glucose isomerase in the fixed form, suspended in a given volume of D-glucose solution in cases studied, is about the same as when the same quantity of D-glucose isomerase in the soluble form is dissolved in that volume of the solution of D-glucose. 

The course of the isomerization reaction observed in a suspension of the fixed isomerase in D-glucose solution is translatable to the degree of isomerization attainable as a function of the rate of flow of the substrate through a bed of the fixed isomerase. In adapting the rate expressions 4 and 6 to a reactor in which substrate flows continuously through a bed of fixed enzyme, the factor t expresses the ratio of the volume of substrate in contact with enzyme in the reactor to the volumetric rate of flow. 

When maximum concentration of enzyme is attained in the fermenter, the broth may be filtered, the solid washed thoroughly with water, and the enzyme-containing cell-mass used directly in isomerization reactions. Takasaki and Kamibayashi described a heat-treatment system that was applied to a Streptomyces fermentation-broth before filtration, and that fixed the isomerase within the cell structures by inactivating lytic enzymes that would otherwise cause the isomerase enzyme to be leached out of the cell when added to substrate in the isomerization reactors.44 The cell-fixed isomerase can be used in stirred-tank or fixed-bed reactors through which the D-glucose solution to be isomerized may be passed continuously while the enzyme that is fixed within the cell material is retained in the reactor. 

Phosphorylation and isomerization. Glucose, produced by the digestion of dietary carbohydrate., is first phosphorylated at the hydroxyl group on C6 by reaction with ATP in a process catalyzed by the enzyme hexokinase. The glucose 6-phosphate that results is isomerized by glucose 6-phosphate isomerase to fructose 6-phosphate. As the open-chain structures in Figure 29.4 show, this isomerization reaction takes place by keto-enol tautomerism (Section 22.1), since both glucose and fructose share a common enol ... 

Isomerases Catalyze isomerization reactions Racemases, epimerases, isomerases... 

Figure 2. The isomerization reaction catalysed by triosephosphate isomerase. The enzyme takes the ketone dihydroxyacetone phosphate to the aldehyde D-glyceraldehyde-3-phosphate. Evidence for an enediol intermediate comes in part from the side-reaction that also occurs the enzyme produces methyl glyoxal and inorganic phosphate.

E. coli possesses two p-hydroxyacyl-ACP dehydratases. One is encoded by fabZ, and is active on all chain lengths of saturated and unsaturated intermediates. This enzyme is distinct from the dual-function P-hydroxydecanoyl-ACP dehydratase/isomerase (encoded by fabA) first described by Bloch and coworkers. The FabA enzyme dehydrates saturated, but not unsaturated, fatty acid intermediates and catalyzes a key isomerization reaction at the point where the biosynthesis of unsaturated fatty acids diverges from saturated fatty... 

The half-life for nonenzymic interconversion of the anomers is about 0.4 sec. Stopped-flow kinetic measurements, using the isomerase from yeast, indicate that both the a- and j8-anomers of glucose 6-phosphate are used directly by the enzyme to give the a- and )8-anomers of fructose 6-phosphate, although the a-anomer is consumed at least 20-fold faster than the )3-anomer. In addition, the enzyme is capable of catalyzing the interconversion (anomerization) of the a-and jS-anomers. With the a-anomer of glucose 6-phosphate, the anomerization reaction is approximately twice as fast as the isomerization reaction. 

The distribution of 0 between the hydroxyl and hydroxymethylene functions of 19a and 19b, determined by the fragmentation pattern of these two species during mass spectrometry, could only result from addition of the carboxyl of the enzyme to the a face of the steroid. Had the carboxyl added to the spiro carbon from the jS face, the 0 would have been located in the hydroxymethylene function of 19a. Clearly, if the binding of the oxiranyl steroids is analogous to that of steroid substrates, Asp-38 could not be involved in the normal intramolecular proton transfer associated with the isomerization reaction. However, as recently emphasized by Pollack ei al., the oxiranyl steroids could conceivably bind to the active site in an upsidedown orientation in comparison to that of the substrate steroids (133). In this case, Asp-38 would still be the most probable base involved in the intramolecular proton transfer reaction. Perhaps upsidedown binding accounts for the report that the C-4a hydrogen of 5-androstene-3,17-dione undergoes slow labilization in the presence of the isomerase (134, 135). 

Hydrolases catalyze the cleavage by the addition of water (hydrolysis), while lyases catalyze the cleavage of C-C, C-0, or C-N bonds (addition of groups to double bonds or formation of double bonds by removal of groups). Isomerases according to their name promotes isomerization reactions, e.g. the transfer of functional groups within the same molecule... 

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